1. Field of the Invention
This invention relates to an apparatus for processing an audio signal. In addition, the invention relates to a computer program for processing an audio signal.
2. Description of the Related Art
PCM audio data are generated by subjecting an analog audio signal to analog-to-digital conversion. The PCM audio data inevitably contain quantization errors. When the PCM audio data are converted back to an analog audio signal via digital-to-analog conversion, sounds represented by the resultant analog audio signal are different from those represented by the original analog audio signal due to the quantization errors.
It is known to compress the PCM audio data on an irreversible basis. When the compression-resultant audio data are expanded to PCM audio data and the PCM audio data are converted back to an analog audio signal via digital-to-analog conversion, sounds represented by the resultant analog audio signal are more different from those represented by the original analog audio signal.
Japanese patent application publication number P2002-169597A discloses an audio signal processing apparatus designed to improve the quality of sounds represented by an audio signal. The apparatus in Japanese application P2002-169597A includes a sampling-rate conversion circuit which receives first PCM audio data having a sequence of samples with a first sampling frequency. The sampling-rate conversion circuit implements up-sampling of the first PCM audio data. Specifically, the sampling-rate conversion circuit periodically samples the first PCM audio data at a second sampling frequency which is equal to twice the first sampling frequency. Thereby, the sampling-rate conversion circuit changes the first PCM audio data into second PCM audio data having a sequence of samples with the second sampling frequency. A digital low pass filter removes high-frequency noise (aliasing) components from the second PCM audio data to get third PCM audio data. A polarity detection circuit senses waveform extremes, that is, waveform polarity inversions, represented by the third PCM audio data. The polarity detection circuit calculates how many times the interval between the moments of the occurrence of two neighboring sensed waveform extremes is larger than a sampling period corresponding to the second sampling frequency. In other words, the polarity detection circuit calculates the number of samples which exist during every half-wave interval defined between waveform extremes. The polarity detection circuit outputs data representative of the result of the calculation to a correction table. The correction table stores a set of predetermined weighting signals assigned to different states of the calculation-result data fed from the polarity detection circuit. The correction table selects one from the weighting signals which corresponds to the state of the present calculation-result data. The correction table outputs the selected weighting signal to an output data generation circuit.
The apparatus in Japanese application P2002-169597A further includes a difference calculation circuit which computes the difference between the levels represented by two successive samples in the third PCM audio data outputted from the digital low pass filter. The difference calculation circuit generates data representative of the computed difference. The difference calculation circuit outputs the difference data to the output data generation circuit. The output data generation circuit receives the third PCM audio data from the digital low pass filter. The output data generation circuit produces a fourth PCM audio data in response to the third PCM audio data, the weighting signal, and the difference data. Specifically, the output data generation circuit multiplies the difference data by the weighting signal to generate corrective data. The output data generation circuit adds the corrective data to the third PCM audio data, and thereby corrects the third PCM audio data into the fourth PCM audio data. Basically, the fourth PCM audio data are improved over the first PCM audio data in sound quality.